Effectiveness of XP-Endo Finisher and Passive Ultrasonic Irrigation in the Removal of the Smear Layer Using two Different Chelating Agents

Statement of the Problem: The smear layer may harbor microorganisms and necrotic pulp tissue, jeopardizing irrigant penetration. Recently, Dual Rinse®, a weak chelating agent, has been introduced to the market. However, its chelating capacity in the final irrigation protocol with different activation systems has not yet been deeply analyzed. Purpose: The aim of this ex vivo study was to evaluate the effectiveness of passive ultrasonic irrigation (PUI) and XP-endo Finisher (XP) on smear layer removal in combination with two chelating agents, ethylenediaminetetraacetic acid (EDTA) and etidronic acid (HEDP). Materials and Method: This in vitro, experimental study evaluated fifty-two single-rooted human teeth were standardized to 16 mm in length. Root canal instrumentation was performed by the ProTaper Gold system up to the F4 file. The apical end of the samples was sealed with wax to simulate a closed system. Teeth from group 1 (n=24) were irrigated with 3% sodium hypochlorite (NaOCl) and 17% EDTA, while teeth from group 2 (n=24) were irrigated with 3% NaOCl mixed 9% HEDP. Both groups were divided into two subgroups (n=12) depending on the activation system used: XP (group XP-EDTA and XP-HEDP) or PUI (group PUI-EDTA and PUI-HEDP). The specimens were evaluated by scanning electron microscopy at 3, 5 and 8mm from the apex. Statistical analysis was performed using ANOVA and Bonferroni tests considering p> 0.05 as significant. Results: PUI-EDTA was the most effective at removing the smear layer, with a statistically significant difference from XP-EDTA (p< 0.042) and group XP-HEDP (p< 0.003). There were no statistically significant differences among the other groups. Conclusion: Under the conditions of this ex vivo study, no activation system was able to completely remove the smear layer from the root canal walls. However, the combination of NaOCl with ultrasonically activated EDTA obtained better results than the other treatments.


Introduction
An irregular layer containing microorganisms, organic and inorganic remnants and odontoblastic processes is formed by action of instruments that covers the instrumented walls [1]. This smear layer reduces the ability of the irrigants to penetrate the dentinal tubules [2] and may prevent the adaptation of filling materials to the dentinal walls [3]. In addition, it may serve a nutrient for residual bacteria inside the root canal system after endodontic procedures, thus promoting treatment failure [4].
Although sodium hypochlorite (NaOCl) has most of the properties required for an irrigant, it is not capable of removing the inorganic part of the smear layer [5].
The chelators, such as ethylenediaminetetraacetic acid (EDTA) and citric acid, may play an important role in smear layer removal during endodontic treatment [6].
Although both solutions are capable of removing the smear layer, citric acid has a higher capacity at equal concentrations [7]. Chelating agents react with the calcium ions of the debris that are formed during the instrumentation of the canal, keeping the particles in suspension and facilitating their removal.
During instrumentation, it is recommended to avoid mixing NaOCl with the chelating agent because their combination can weaken dentin and affect its integrity [8]. Furthermore, the mixture of NaOCl+EDTA decreases the amount of available chlorine and consequently the antimicrobial capacity of the NaOCl solution [9][10]. Recently, a weaker chelator known by the name etidronic acid (HEDP) has been introduced to the market (Dual Rinse, Medcem, Weinfelden, Switzerland). HEDP mixed with NaOCl is capable of maintaining the properties of chlorine, does not affect the dissolving capacity, does not decrease the antimicrobial activity and simultaneously removes the smear layer [11][12][13].
Positive pressure syringe irrigation is the most commonly used irrigation system [14]. Due to its limitations [15][16][17] and difficulties in addressing complex morphologies [18] it is recommended to combine syringe irrigation with an activation system to increase the  [26][27], dissolving organic tissue from artificial cavities in combination with NaOCl [28] and penetrating into the isthmus [29] has been analyzed. In addition, the capacity of XP in conjunction with different irrigants to the remove the smear layer and debris has been studied [30][31][32][33][34]. However, its combination with a weak chelator has not been analyzed deeply, particularly adjunct with HEDP and a standardized and constant flow rate monitoring. For that reason, this ex vivo study aimed to evaluate smear layer removal using a standardized flow rate with two different chelating agents (EDTA and HEDP) and two different activation techniques (XP and PUI).

Materials and Method
This study was carried out taking into consideration the The minimum sample size was established at n=12.
The instrumentation was carried out by an experienced endodontist and methodology was carried out according to 2 previously described protocols [36][37]. magnification. The area corresponding with the greatest amount of smear layer was photographed [37]. Each root third was evaluated for smear layer removal by two experienced examiners based on the following scale [35]: 0, all the tubules were visible; 1, more than 50% of the tubules were visible; 2, less than 50% of the tubules were visible; s 3, no tubules were visible.
The mean and standard deviation were then calculated for each group and statistical analysis of the data was carried out by SPSS 21.0 (SPSS Inc, Chicago, IL) program. ANOVA and Bonferroni tests were used, with statistical significance established at p<.05.

Results
The interexaminer agreement was 87.3%, as demonstrated the kappa test. The mean scores and standard deviation of all tested groups are reported in Table 1.
Scanning electron microscopy images taken after the different irrigation protocols in the different root canal

Discussion
The aim of this research was to analyze the effectiveness of two different chelating agents activated with two different techniques in the elimination of the smear layer. Although it is difficult to standardize irrigation with two chelating agents that are applied at different treatment times, in the present study, the same amount ages other agents such as citric acid [40]. HEDP has been introduced recently as a weaker chelating agent that can be used during root canal instrumentation due to its compatibility with NaOCl [12] and has demonstrated promising results [33]. However, its efficacy in the smear layer removal during chemomechanical preparation and in conjunction with different activation devices has not been analyzed. It must be taken into consideration that the effects of HEDP are dependent on concentration, so higher concentrations could produce different results [11].
The findings of the present study revealed that the null hypothesis may be rejected. They confirmed those published previously demonstrating that HEDP had a lower chelating capacity than EDTA [7,11]. In fact, under the conditions of this study, the use of HEDP during chemomechanical preparation does not prevent smear layer formation.
However, the debate about the ideal chelator remains open, taking into account that the strong chelators, such as citric acid and EDTA, present certain disadvantages. Strong chelators may compromise the mechanical integrity of dentine and erode the dentinal tubules [41][42] and may negatively affect the antimicrobial and solvent properties of NaOCl if they are mixed [12]. Moreover, they are not able to completely eliminate the accumulated debris [43], and an increase in preparation errors has been reported when they were used during instrumentation [44]. Therefore, the use of HEDP during the instrumentation step [33] and a final activation using a strong chelating agent may represent an interesting combination to prevent the storage of debris and eliminate the smear layer more effectively.
Future studies are necessary to investigate the efficacy of this mixing on the reduction of accumulated debris and elimination of the smear layer. However, to be able to use only one irrigant during the entire treatment process, future studies should also evaluate whether a longer final activation time of HEDP may be able to achieve the same efficacy as EDTA.
Recently, XP has been introduced as a supplementary technique to be used as a final step to improve the efficacy of root canal cleaning and disinfection [45].
Although a final activation using PUI may improve root canal cleaning [36], this technique is not able to completely clean and disinfect the entire root canal system [43].  [31] showed no differences between PUI and XP in the removal of debris, but without the consideration of the usage of weak chelating agents during instrumentation or the activation of strong chelating agents such as EDTA in the final irrigation.
The activation time for this study was 30 seconds for EDTA and NaOCl based on previous reports [36,48].
Similarly, the NaOCl and HEDP mixture was activated for 60 seconds but refreshed after 30 seconds.
The finding found in this study showed that, regardless of the irrigation protocol used, the coronal part of the canal was always cleaner than the apical and middle thirds. Some studies agree with this discovery and could be attributed to the larger diameter of the canal in the coronal third, which exposes dentine to a higher volume of irrigants and facilitating making smear layer removal by increasing the effectiveness of the activation systems [49].

Conclusion
In summary, using a standardized flow rate of 3mL/min in combination with 17% EDTA as the chelating agent and PUI as the activation modality was significantly more effective in the removal of the smear layer from the root canal walls, but none of the irrigation methods assessed was able to completely remove the smear layer.